Wake Dynamics of a Wind Turbine with an Oscillating Rotation Rate at High Reynolds Numbers

Understanding the wake behavior of wind turbines under unsteady conditions is critical for optimizing wind farm power generation. Dynamic induction control is a method which imposes unsteady thrust conditions on turbines, which could allow the wake to recover more quickly than the static case. Increased wake recovery can contribute to an improvement in the overall power density of a wind farm. The effect of dynamic induction control on the wake dynamics of a horizontal axis wind turbine is experimentally studied at a Reynolds number of $4\times 10^6$, and periodic oscillations of the tip-speed ratio are forced at Strouhal numbers of 0.15, 0.25, and 0.4. Force and velocity measurements are taken to characterize the turbine and its wake. Streamwise and spanwise velocity sweeps are conducted using hot-wire anemometry. While the thrust is observed to vary sinusoidally, there is no significant effect on downstream wake recovery. Instead, results show a traveling wave of velocity fluctuations propagates downstream through the wake. We examine two mechanisms for the generation of the traveling wave: the tip-speed ratio oscillation and the change in location of the breakdown of the helical tip vortices. The phase of the thrust coefficient affects the mechanism of traveling wave generation. These results help to establish the dynamic wake behavior of wind turbines under unsteady conditions at high Reynolds numbers.